State of the art in automotive OEM finishing includes two-coat coating systems composed of a basecoat material and a clearcoat material, also referred to collectively as topcoat materials. The purpose of these coatings systems is to protect the coats situated beneath them and to endow the overall paint system with the desired optical properties and the requisite weather resistance. Basecoat materials comprise pigments to impart color and/or effect. Clearcoat materials protect the basecoat and intensify its optical effect. Topcoat materials may be solventborne or aqueous.
Waterborne urethane-based two-coat coating systems are known in the art (BASF Handbuch Lackiertechnik, Vincentz Verlag, 2002, pp. 98-99). They may be based on aqueous polyurethane dispersions and on hybrid polyacrylate-polyurethane resins (Winnacker, Küchler, Chemische Technik—Prozesse und Produkte, Volume 7—Industrieprodukte, Wiley VCH, 2004, pp. 662-669).
Aqueous polyurethane dispersions can be prepared by emulsifying hydrophobic polyurethanes in water with the aid of external emulsifiers. Also possible, however, is to make the polyurethane self-dispersible by incorporating hydrophilic groups. Dispersibility in water can be achieved, for example, through carboxylate groups which have been converted, using amines, into anionic groups (anionic, polyurethane dispersions). It is also possible to obtain dispersibility in water through reaction of tertiary amino alcohols with prepolymers which still contain free isocyanate functions, followed by neutralization with acid (cationic polyurethanes). A further alternative is that of modifying prepolymers having free isocyanate functions with water-soluble long-chain polyethers (nonionic polyurethanes). Aqueous dispersions of polyurethanes can be prepared by a variety of methods, which are described in Winnacker, Küchler, Chemische Technik—Prozesse und Produkte, Volume 7—Industrieprodukte, Wiley VCH, 2004. These methods include the acetone method, the prepolymer mixing method, and the melt dispersion method.
The aim of using hybrid polyacrylate-polyurethane resins is to combine the positive properties of the two types of polymer employed and to minimize the negative properties. Depending on type they are differentiated as hybrid systems generated by cleanly mixing the different dispersions (blends), hybrid systems in which a chemical connection exists, at least to a certain extent, between the different kinds of polymer, and hybrid dispersions in which the different classes of polymer form interpenetrating networks (IPNs).
Aqueous coating systems based on the stated hybrid polyurethane-polyacrylate dispersions are already known and widespread within the paint industry. Typically the hybrid polyurethane-polyacrylate dispersions are prepared by emulsion-polymerizing a vinylpolymer (“polyacrylate”) in an aqueous polyurethane dispersion. It is also possible, however, to prepare the hybrid polyurethane-polyacrylate dispersions as secondary dispersions.
The term “secondary dispersions” refers to those aqueous dispersions which to start with are polymerized in a homogeneous organic medium and thereafter are redispersed in an aqueous medium with neutralization, generally without the addition of external emulsifiers.
WO-A 95/16004, for example, describes water-thinnable film-forming binders based on oligourethane-acrylate copolymers. In that case a monomer mixture of vinylically unsaturated monomers is subjected to free-radical polymerization in a water-dilutable organic solvent and in the presence of a water-soluble oligourethane having a molecular mass of 750 to 1000. This secondary dispersion is then used to formulate baking varnishes.
DE-A 40 10 176 discloses oxidatively drying coating materials using as their binder a polymer which is obtainable by subjecting ethylenically unsaturated monomers to polymerization in an organic solvent in the presence of a polyurethane resin that contains polymerizable double bonds.
EP-A 657 483 describes aqueous two-component coating materials composed of a polyisocyanate component and an aqueous polyurethane dispersion as polyol component. The polyol component is prepared by neutralization and dispersion of unsaturated polyurethane macromers hydrophilicized with acid groups and containing lateral and/or terminal vinyl groups. These PU macromers, where appropriate following addition of further vinylic monomers, are then subject to free-radical polymerization in an aqueous phase.
EP-A 742 239, finally, discloses two-component coating systems based on polyisocyanate crosslinkers and aqueous, hydroxy-terminated polyurethane prepolymers/acrylic hybrids. These hybrid polymers are obtained by reacting a water-dispersible NCO-functional urethane prepolymer with at least one hydroxy-functional acrylate monomer and an alkanolamine to form a hydroxy-functional urethane prepolymer/monomer mixture, which is dispersed in water. A free-radical initiator and a hydroxy-containing chain extender are then added to this dispersion, after which the aqueous reaction mixture is heated, and the free-radical polymerization of the acrylate monomers and the chain extension step of the polyurethane are carried out and completed. The resultant hydroxy-functional polyurethane prepolymer/acrylic hybrid dispersions can then be formulated into the ready-to-use two-component coating compositions by incorporating hydrophilicized polyisocyanates with stirring. A disadvantage here is the use of up to 6% of molecular weight regulators, based on acrylate monomer (e.g., thiols), which may adversely affect important paint properties such as resistance properties and film hardeners.
Secondary hybrid polyurethane-polyacrylate dispersions suitable for preparing two-component (2K) coating compositions are disclosed in DE-A 103 08 103. The hybrid polyurethane-polyacrylate dispersions described there are prepared from a polyurethane having a molecular weight Mn of 1100 to 10 000 and from one or more vinylically unsaturated monomers, which are free-radically polymerized in a homogeneous, nonaqueous phase, undergo at least partial neutralization, and are then dispersed in an aqueous phase. They are suitable for a variety of fields of use.
Used as two-coat coating systems on polymeric substrates in particular, however, these two-component coating compositions lack adequate adhesion: on polymeric substrates which are coated directly, without adhesion promoter, with the coating system in the form of an aqueous basecoat material, direct adhesion is found to be lacking. Additionally, the adhesion of the coating systems, as aqueous basecoat materials, to polymeric substrates coated with adhesion promoters fails to meet the requirements. Moreover, following build-up with a waterborne clearcoat material, aqueous basecoat materials based on this system undergo swelling, as is evident, among other things, after a 100-hour immersion test at 40° C. Furthermore, they or their aqueous clearcoat materials do not exhibit the combined profile of properties desired on polymeric substrates, namely the combination of a high flexibility with to good wash-brush resistance and good chemical resistance.
It is an object of the invention, therefore, to provide an aqueous clearcoat material for producing a waterborne urethane-based two-coat coating system, said coating material satisfying the typical requirements imposed on coating systems, more particularly the requirements concerning the gloss (a gloss value of greater than 85) and concerning the development of haze. A further intention is that an aqueous clearcoat material should be provided for producing a waterborne urethane-based two-coat coating system which on plastics displays a combined profile of properties with high flexibility in conjunction with good wash-brush resistance and good chemical resistance.
It is a further object of the invention to provide an aqueous basecoat material which is intended for producing a waterborne urethane-based two-coat coating system which on all substrates, more particularly polymeric substrates, exhibits effective adhesion with or without adhesion promoter. It is a preferred object of the invention, furthermore, to provide a corresponding aqueous basecoat material which, after build-up with a waterborne clearcoat material, exhibits only slight swelling.
It is also the aim that a waterborne urethane-based two-coat coating system be provided which meets the requirements imposed on the aqueous clearcoat material and the requirements imposed on the aqueous basecoat material. In addition the two-coat coating system provided ought to have good processing properties, i.e., a high circulation stability in particular.